01 - Combine breadcrumbs and milk in a large mixing bowl; allow to soak for 5 minutes.
02 - Add ground beef, ground pork, onion, garlic, egg, salt, black pepper, allspice, and nutmeg to the soaked breadcrumbs. Mix gently until ingredients are just combined.
03 - Form the mixture into walnut-sized balls, each approximately 1 1/4 inches in diameter.
04 - Heat butter and oil in a large skillet over medium heat. Cook meatballs in batches, turning them until browned on all sides, about 8 minutes. Remove browned meatballs and set aside.
05 - Melt butter in the skillet. Add flour and whisk continuously for 1 minute to form a roux.
06 - Gradually whisk in beef or vegetable stock until the mixture is smooth. Stir in heavy cream, soy sauce, and Dijon mustard. Simmer the sauce for 5 to 7 minutes, stirring, until thickened.
07 - Return meatballs to the skillet and coat with sauce. Allow to simmer gently for 5 to 10 minutes, ensuring meatballs are heated through.
08 - Serve hot, accompanied by mashed potatoes, lingonberry preserves, and steamed vegetables if desired.